The invention relates generally to the field of solar power, and more specifically to communication networks used to monitor and control photovoltaic solar power arrays.
In a conventional photovoltaic (PV) solar power array, the overall system efficiency is often significantly degraded by partial shading of the array, damaged cells, or mismatches in the characteristics of individual cells. To explain how this occurs, FIG. 1 shows a conventional system 1 consisting of four strings 2 wired in parallel. Each string consists of a number of solar modules 3 in series, and a blocking diode 4. The array feeds power to an inverter 5 via a bus with positive 6 and negative 7 rails. Ideally, the characteristics of each solar module 3 are identical to all the others; given equal sunlight, each solar module 3 outputs the same voltage, and each string 2 carries the same current. In theory, this allows all the PV cells in the array to simultaneously operate at their maximum power points. But in reality this is never the case. For example, suppose one of the solar modules 3 in the first string is shaded. This lowers its output voltage, and hence the voltage of the entire string 2. The other three strings are forced to the same voltage because the strings are wired in parallel. The maximum power point tracking system in the inverter 5 is then forced to a point that is suboptimal for all PV cells. So a single shaded or damaged PV cell can prevent every PV cell in the array from operating at its maximum power point.
One solution well known in the art is to monitor the performance of each module 3 in the array so problems can be identified and corrected. The performance of a module 3 is measured by various parametric data, such as the modules output voltage. This requires some form of communication network to convey parametric data from each solar module 3 to a central computer. FIGS. 2A-2C show three well known methods to implement such a communication network. In all three examples, a plurality of Solar Modules 20 (SM0-SMN) are connected to an inverter 21 via a power bus 22, and a computer 23 receives the parametric data. In FIG. 2A the communication network is implemented with dedicated wires 24. In FIG. 2B the network is implemented with radio links 25. And in FIG. 2C the network is implemented via Power Line Communications (PLC) using a current transformer 26 such as a Rogowski coil.
All the examples in FIGS. 2A-2C are similar in that they use a bus topology for the network, where each solar module 20 communicates directly with the computer 23 via a shared medium, or bus, and the modules 20 must transmit data one at a time. For the system to work, each solar module 20 must be assigned a unique network address so the computer 23 can keep track of which module 20 each piece of data comes from.
When the parametric data indicates that a failure has occurred, the computer 23 identifies the failed solar module 20 by its network address. But a network address is often just a random number programmed into each module 20 at the factory, which is not very helpful to the repairman who must replace the failed module; he needs to know the physical location of the module 20 on the rooftop. So someone has to make a map that relates the network address of each module 20 to its physical location. This task is usually done by the people who install the solar array, and the extra labor adds to the installation cost.
Some prior art references have attempted to solve this problem. For example, U.S. Publication No. 2009-0140719 discloses a system where the solar modules automatically assign their own network addresses based on their location within a chain of series-connected network cables. For instance, the computer might tell the repairman that the bad module is “third from the end of the chain”. If the repairman can see the network cables, then he can trace the chain back to the failed module, but unfortunately the cables are typically hidden underneath the modules.
Accordingly, there is a continuing need in the field of solar power for a communication network that simplifies the installation process by avoiding the task of creating a map that relates the network address of each solar module to its physical location. The present invention fulfills this need and provides other related advantages.